Excretion is the process by which animals rid themselves of waste products and of the nitrogenous by-products of metabolism. Digestion is sequence by which food is broken down and chemically converted so that it can be absorbed by the cells of an organism.
In order for nutrients and minerals to be absorbed into blood, food we eat must be broken down into smaller molecules. The digestive system breaks down ingested food and liquid into smaller molecules our body can absorb. In turn, the nutrient-rich blood is then distributed throughout the body by the cardiovascular system.
If the human body is to operate correctly, it must be able to get rid of waste and nonessential items. Several organ systems participate in the excretion of waste products. For instance, the urinary system disposes nitrogenous waste while the digestive system rids the body of indigestible food as feces.
Through excretion organisms control osmotic pressure—the balance between inorganic ions and water—and maintain acid-base balance. The process thus promotes homeostasis, the constancy of the organism’s internal environment.
See Anatomyandphysiologyi.com's Maintaining Life: Necessary Life Functions to learn more.